larger the ion the more is the ionic mobility. then why the mobility of rubidium is more than that of caesium when Cs is a larger ion

The salt metal particle exist as hydrated particles M+(H2O)n in the fluid arrangement. The level of hydration, reductions with ionic size as we go down the gathering. Henceforth Li+ particle is generally hydrated e.g. [Li(H2O)6]+. Since the portability of particles is contrarily relative to the extent of the their hydrated particles 

ionic versatility decrases with increment in nuclear size 

in soluble base metal in aaqueos arrangement measure arrange invert 

so request of ionic versatility request is 

cs+ then rb+ > then k+ then na+